Prosthetic devices are artificial devices used to replace or strengthen a particular part of the body. Such devices can be used in humans or animals to repair or replace diseased or damaged bone, allied tissue associated with the bone, and/or joints associated with the bone. Primarily, prosthetic devices are used to correct or prevent skeletal deformities or injuries and to alleviate the pain and discomfort associated with the deformities or injuries.
When implanting a prosthesis, typically a receiving site or cavity is first prepared in an adjoining bone. In particular, the bone can be cut and reamed out in order to accommodate the prosthesis. A bone cement is then mixed and placed in the receiving site or cavity. A prosthesis is positioned in the bone cement, and the bone cement is subsequently cured and hardened affixing the prosthesis to the bone.
In most applications, bone cement is made from an acrylic polymeric material. Typically, the bone cement is comprised of two components: a dry power component and a liquid component, which are subsequently mixed together. The dry component generally includes an acrylic polymer, such as polymethyl methacrylate (PMMA). The dry component can also contain a polymerization initiator such as benzoyl peroxide, which initiates the free-radical polymerization process that occurs when the bone cement is formed.
The liquid component, on the other hand, generally contains a liquid monomer such as methyl methacrylate (MMA). The liquid component can also contain an accelerator such as an amine (e.g., N,N-dimethyl-p-toluidine). A stabilizer, such as hydroquinone, can also be added to the liquid component to prevent premature polymerization of the liquid monomer.
When the liquid component is mixed with the dry component, the dry component begins to dissolve or swell in the liquid monomer. The amine accelerator reacts with the initiator to form free radicals which begin to link monomer units to form polymer chains. In the next two to four minutes, the polymerization process proceeds changing the viscosity of the mixture from a syrup-like consistency (low viscosity) into a dough-like consistency (high viscosity). Ultimately, further polymerization and curing occur, causing the cement to harden and affix a prosthesis to a bone.
Once implanted, a prosthetic device ideally closely assimilates the characteristics of the bone and/or the joint that the device is intended to repair or replace. The implanted prosthetic device should be capable of supporting and withstanding stresses and strains normally imparted to the repaired or replaced bone joints.
The above process for implanting a prosthetic device is generally accepted within the art and has proven to be a successful process for repairing or replacing damaged bones, bone joints and the like. Prosthetic devices, however, can be prone to loosen within the bone cavity over time. In particular, the acrylic bone cement, which is neither as strong nor as viable as bone tissue, has been universally considered the weakest link in the implant design. It has been found that the bone cement can break away from the prosthesis, can break away from the bone, or can develop stress or fatigue cracks when repeatedly exposed to the normal stress and strains supported by the bones.
Due to these problems, attempts have been made to improve the mechanical properties of prosthetic devices and of the cement interface that exists between the device and the bone. For instance, U.S. Pat. No. 4,491,987, which was filed by the current inventor and which is incorporated herein in its entirety by reference, discloses an improved prosthesis and process for orthopedic implantation of the prosthesis. The current inventor's prior patent is generally directed to a prosthesis precoated with a polymeric material that is compatible with bone cement. Once implanted, the precoat provides a stronger interfacial bond between the bone cement and the prosthesis.
The present inventor's prior work provided great advances in the art with respect to the implantation of orthopedic devices, namely orthopedic devices made from metals such as stainless steel, titanium, and cobalt chrome alloys. However, although metallic devices have achieved relatively high degrees of success in repairing joints, these devices are not always well suited for every application. For instance, in some applications, it is preferred to use a more flexible and less rigid material than metal for opposing joint structures. Specifically, polymeric prosthetic devices are particularly well suited for use in replacing the acetabular cup in a hip replacement and replacing the tibia plateau in knee replacements.
Unfortunately, high strength polymeric materials, such as ultra high molecular weight polyethylene (UHMWPE), do not adhere well to conventional bone cement materials. Thus, in order to attach polymeric prosthetic devices to an adjoining bone using bone cement, deep grooves have been formed into the prosthetic devices for forming a mechanical interlock with the bone cement.
In other prior art constructions, polymeric prosthetic devices include a metal backing and stem for bonding the devices to a bone using a bone cement. Alternatively, the polymeric devices have been installed into a bone without cement using bone screws. Bony tissue ingrowth has also been proposed in the past as a means for joining a prosthesis to bone.
Thus far, however, these prior art methods and constructions for polymeric prosthetic devices have not proven to be completely successful. Thus, a need exists for a process for implanting a polymeric prosthesis into a prepared area of the body. More particularly, a need exists for a process that strengthens the interface between a bone cement and a polymeric prosthesis for decreasing the likelihood that the prosthesis will loosen and break away from the cement over time. Further, a need also exists for a precoated polymeric prosthesis that will readily adhere to a curing bone cement mixture once implanted into an adjoining bone.